Fiber reinforced thermoplastic or thermoset material, commonly referred to as thermoplastic or thermoset prepreg, or simply, prepreg, is used in the construction of structural parts for various devices. Prepreg has the advantages of being strong and rigid, yet lightweight, and thus, prepreg is used to make devices for aerospace, military, electrical and computer science applications.
Prepreg materials generally consist of carbon or glass fibers impregnated within an epoxy or thermoplastic resin that is often formed into unidirectional rolled goods or other shaped ply. The fibers within the ply are oriented according to the device being constructed to provide the various structural parts of the device with the requisite strength and rigidity. Ply orientation is determined by the cut of the material. A ply is strongest in the direction of its fiber alignment. Thus, a portion of the fibers is aligned parallel to the direction in which strength is required by the structural part.
Usually, several plies with different, but specifically designed, fiber orientations are assembled into a multi-ply assembly. Moreover, several multi-ply assemblies are then stacked on top of each other to compose a laminate, or layup, with a specific orientation of fibers to bolster the structural integrity of the part under construction. The layup is then cut into composite pieces and assembled into a prepreg wedge to form a particular structural part of the device under construction. Assembly of the prepreg wedge requires stacking and welding the composite pieces in sequence and orienting the pieces according to a geometry envelope. Typically, a plurality of prepreg wedges is compression molded to form one composite segment of the part under construction and several composite segments are assembled to form the entire part. One area where segment molding is used frequently is in the construction of segmented cylinders.
For some useful applications, including composite sabots, compression molding of a segmented cylinder generally requires that sufficient prepreg wedges be loaded into a mold cavity to form a 120-degree composite segment of the cylinder, wherein three 120-degree composite segments form the entire cylinder. Subsequently, the mold cavity is heated and a press applies pressure to mold the prepreg wedges together. Heat from the mold cavity brings the thermoplastic or thermoset of the prepreg to its melting point. As the thermoplastic or thermoset melts, it flows and carries the composite fibers with the flow. However, as stated hereinabove, the prepreg kits have been structurally designed with specific fiber orientations and the fiber orientations must be maintained during segment molding to maintain the structural integrity of the part. If the mold that is holding and compressing the prepreg wedges applies force at the wrong time or in the wrong direction, the result is an unacceptable part with inadequate structural integrity.
Several methods have been attempted to mold cylinders using segmented compression molding of prepreg wedges. One conventional method uses a solid press, or solid ram, that matches the geometry of the cylinder segment to exert vertical pressure across the entire segment, including the center and opposing radial flanks of the cylinder segment. However, the vertical downward pressure from the solid ram across the center of the segment sometimes causes pressure in the wrong direction at the wrong time, resulting in misalignment of the fibers. The solid press also causes the wedges to warp and create air pockets and voids.
One apparatus and method known in the art is one that uses a hinged wing mold to compress the cylindrical segment such as is described in U.S. Pat. No. 5,635,660 entitled "Sabot Segment Molding Apparatus," to McGovern. McGovem's hinged wing mold comprises a mold channel and a hinged wing assembly, wherein a plurality of wedges are loaded into the mold channel. The hinged wing assembly comprises a pair of moveable articulated wings and a wing hinge, wherein wing hinge is secured coaxially along the length of mold channel to prevent downward vertical pressure to wedges. When the hinged wing assembly, motivated by a pressure cylinder, applies force to wedges to mold a segment the moveable articulated wings rotate about the hinge pin towards each other.
Another alternate method known in the art is one that uses a bag wing mold to compress the cylindrical segment. FIG. 2 shows a bag wing mold 200 of the prior art designed by Alliant Techsystems Inc. Bag wing mold 200 comprises a mold channel 220 and a bag wing assembly 250, wherein a plurality of wedges 210 is loaded into mold channel 220. Bag wing assembly 250 comprises a pair of moveable articulated wings 240 and a bag hinge 242, wherein bag hinge 242 is secured coaxially along the length of mold channel 220 to prevent downward vertical pressure to wedges 210. Bag wing assembly 250, motivated by a pressure cylinder 202, applies force through pressure block 204 to prepreg wedges 210 to mold a segment 212.
As demonstrated above, the design of a compression mold is critical to the production of a structurally superior segmented cylinder. Unfortunately, compression molds of the prior art, some using moveable articulated wings, sometimes do not apply forces in a correct or timely manner. For example, devices of the prior art may exert undue downward pressure across the center of a formed segment, or unequal pressure to the opposing radial flanks of the segment. Such undesirable misapplications of pressure may cause anomalous compression, resulting in misalignment, bending, or breakage of the composite fibers within the prepreg. Misaligned, bent, or broken fibers all contribute to the structural degradation of the segment. Further, use of such prior art compression molds may exert pressure in such a way that may also cause the prepreg plies and wedges of a segment being formed to warp and create air pockets or voids within the segment. Warpage and voids also contribute to the structural degradation of the segment.